The recent development of a specific radioimmunoassay for amphibian (bullfrog, Rana catesbeiana) thyrotropin (TSH) has made it possible to study the effects of various neuropeptides on the release of TSH from the pituitary in vitro. Up to now, corticotropin-releasing factor of bullfrog origin has been shown to have a potent TSH-releasing activity, whereas gonadotropin-releasing hormone and TSH-releasing hormone exhibit a moderate TSH-releasing effect on the adult, but not larval, pituitary. In the present study, the effects of pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP), and somatostatin (SS) on the in vitro release of TSH from the bullfrog pituitary were investigated. Both frog (R. ridibunda) PACAP-38 and PACAP-27 caused a concentration-dependent stimulation of the release of TSH from dispersed pituitary cells during a 24-h culture. The PACAP-38- and PACAP-27-induced TSH release was suppressed by a simultaneous application of PACAP6-38. Application of high concentrations of PACAP6-38 alone caused a slight but significant stimulatory effect on the release of TSH. Frog VIP also stimulated TSH release from pituitary cells concentration-dependently. Frog SS1 (homologous to mammalian somatostatin-14) and SS2 (homologous to mammalian cortistatin) did not affect the basal release of TSH but caused a concentration-dependent suppression of the PACAP-38-induced release of TSH. These results suggest the involvement of multiple neuropeptides in the regulation of the release of TSH from the amphibian pituitary.
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